The present invention relates to a maleimide-based copolymer having a high glass transition temperature and being useful as a material and the like for improving various properties such as heat resistance and so forth by combining it with a molding material or thermoplastic resin. This copolymer is usually obtained by a process comprising removing volatile components (devolatilization) from a polymer composition which contains a maleimide-based copolymer and volatile components such as a solvent, an unreacted monomer, volatile by-products or volatile impurities. Therefore, the present invention also relates to a process like this.
It has been known that a maleimide-based copolymer is a thermoplastic resin having the high heat deflection temperature and the high thermal decomposition temperature and, in general, the copolymer is used to elevate heat resistance, impact resistance and molding properties of articles in which other thermoplastic resins are used.
The maleimide-based copolymer is usually produced by copolymerizing a maleimide-based monomer with another monomer capable of copolymerizing with the maleimide-based monomer.
Physical properties of the maleimide-based copolymer are affected by proportion of the maleimide-based monomer unit in structure and, if the proportion is low, heat resistance becomes low, so that a resin composition may not obtain sufficient heat resistance and also, if the proportion is too high, molding and processing properties become worse and impact resistance may decrease.
A copolymer having relatively small proportion of the maleimide-based monomer unit is generally prepared by emulsion polymerizing or suspension polymerization. According to the emulsion polymerization, a maleimide-based copolymer having high proportion of the maleimide-based monomer unit is hard to soften and very difficult to recover from an emulsion and, therefore, a copolymer having low proportion of the maleimide-based monomer unit has been produced. Then, a copolymer obtained from the emulsion polymerization is not preferable because it shows low impact resistance by an effect of a residual emulsifier etc. and badly colors in the course of molding. In the case of suspension polymerization, an aromatic vinyl-based monomer and a maleimide-based monomer are liable to form an alternating copolymer and, therefore, when a copolymer having high proportion of the maleimide-based monomer unit is attempted to obtain, heterogeneous copolymers of different composition are liable to form. In the solution polymerization and bulk polymerization, there have been carried out several attempts to obtain a copolymer in which the amount of unreacted maleimide-based monomer is decreased by a maturing reaction, or to obtain a polymer, in which component distribution in every molecular weight division is narrow and the amount of unreacted maleimide-based monomer is small, by using a defined polymerization process.
The maleimide-based copolymer is obtained in dissolved condition in the case of the solution polymerization or bulk polymerization and obtained in dispersed condition in the case of the emulsion polymerization or suspension polymerization. It is necessary to obtain the maleimide-based copolymer by separating volatile components such as a solvent, an unreacted monomer, volatile by-products or volatile impurities from the above conditions. The reason is that, if the volatile components such as a solvent, an unreacted monomer and so forth remain in the maleimide-based copolymer, there occur deterioration of physical properties such as heat resistance and so forth as well as inferior outside appearance (silver streaks) of a molding, so that physical properties of the molding are much deteriorated.
Because of this, in the case of the emulsion polymerization and suspension polymerization, there has been adopted a method comprising separating polymers by coagulating and filtering processes, steam stripping, and so forth (refer to Japanese Official Patent Provisional Publications No. showa 62-112612 and 62-138510). On the other hand, in the case of solution polymerization and bulk polymerization, the polymers have been separated by a method comprising removing a solvent and an unreacted monomer with extraction by placing the reaction solution, obtained from finish of the polymerization, into another solvent which dissolves the solvent and unreacted monomer but does not dissolve the maleimide-based copolymer to be recovered (refer to Japanese Official Patent Provisional Publication No. showa 58-162616), a method comprising extracting a polymer in melted condition after the unreacted monomer and solvent are evaporated by using a flash evaporator (refer to Japanese Official Patent Provisional Publications No. showa 61-276807, heisei 3-205411, and showa 62-129334), or by other conventional methods.
There has been known a method comprising supplying a reaction mixture resultant from finish of the polymerization to a vent type extruder directly or after drying the mixture by heating beforehand, and thereby forming the polymer into pellets while removing volatile components (while carrying out devolatilization) (refer to Japanese Official Patent Provisional Publications No. showa 59-126411, 59-58006, 57-135814, 50-40688, 57-49603, 50-40687, 63-147501, and heisei 3-49925). Such a method is also used as a method for producing a maleimide-based copolymer (refer to Japanese Official Patent Provisional Publications No. heisei 2-51514 and showa 63-89806).